TORONTO -- Health Canada says it is studying the effectiveness of the so-called "morning after" contraceptive pills in the wake of news they aren't as effective in larger women.

The federal drug regulator says it is assessing new data that suggests a higher body weight could reduce the effectiveness of emergency contraceptives.

Health Canada says it is also looking at whether labelling changes are needed for all brands of the pills to reflect the problem.

The department's statement comes two months after French manufacturer HRA Pharma announced its emergency contraception pill doesn't work in women who weigh more than 176 pounds.

There are four brands of morning-after pills available without prescription in Canada: Next Choice, from Cobalt Pharmaceuticals; HRA Pharma's Norlevo; Option 2, from Perrigo International and Plan B from Women's Health Inc.

Health Canada says if it decides action needs to be taken, it will work with manufacturers to update labels and inform consumers.

The pills contain higher levels of levonorgestrel than standard oral contraceptives and work by preventing ovulation or fertilization of an egg. They can be taken up to 72 hours after unprotected sex or a contraceptive accident such as a condom breaking.

This pills do not have any impact if a woman has already become pregnant.

Health Canada says it is aware of a recent statement from the European Medicine's Agency alerting consumers to the fact that Norlevo's labelling had been altered to make reference to the observation that weight affects the efficacy of the medication.

The added wording says the pill's efficacy is reduced in women weighing 75 kgs or more, and the pill doesn't work for women who weigh more than 80 kgs.